Corrosion sensors

ABSTRACT

A corrosion sensor includes an insulating substrate, a thin film of a first metallic material formed on the substrate; and an array of areas what includes a second metallic material provided at the surface of the thin film. A method for manufacturing such a sensor is also disclosed. An exemplary embodiment is disclosed in which the thin film is patterned to define a number of tracks, the resistance of which can be monitored in order to determine the degree to which the thin film has corroded.

FIELD

This invention relates to corrosion sensors for detecting the action ofcorrosive media acting on a metallic material when mounted in situadjacent a location in the metallic material.

BACKGROUND TO THE INVENTION

Corrosion is a problem which leads to high maintenance and repairoverheads in many different industries. Prompt detection of problemscaused by corrosion is necessary in order for effective mitigationstrategies to be put in place. Various different methods of detectingcorrosion in a metallic material have been developed. Corrosion sensorshave been developed that are able, for example, to detect corrosion, orto monitor the progress of corrosion, or to monitor the degradation ofprotective layers applied to corrosive surfaces.

A simple method to monitor corrosion is the visual inspection of asample of the material of interest. Other sensors have been developedthat are more appropriate for the through life monitoring of astructure. Such sensors can be readily interrogated to provide datarelating to the Many of these known corrosion sensors rely on electricalmeasurements performed on a thin film of metallic material to determinethe level of corrosion that has occurred to a structure. Two known typesof corrosion sensor are described in the Applicant's published EuropeanPatent Applications, Publication Numbers EP1554563 and EP1546679. Theseprior-known sensors comprise patterned conductive thin films formed on asubstrate. The film, which is made of a material that mimics thecharacteristics of the bulk material to which the sensor is attached,defines a plurality of serpentine tracks extending between commonterminals. These sensors can be used as resistive sensors, in which casethe resistance of the sensors is measured over a period of time. Theresult of the action of corrosive media on the tracks is an increase inthe overall resistance of the sensor, as measured between the commonterminals. This measured increase in resistance can then be related tothe effects of corrosion acting on the bulk structure to which thesensor is attached.

A problem that exists in sensors of the above type is that it can bedifficult to relate the sensitivity of the sensor to corrosive media tothe sensitivity of the bulk structure to corrosive media. Where a sensoris used to monitor the progress of corrosion to a bulk structure, it isimportant that the sensor corrodes at a rate at least approximatelyequal to that at which the bulk structure corrodes; or alternativelythat the rate at which the sensor material corrodes can be easilyrelated to the rate at which the bulk structure corrodes. Whilst, in theabove-referenced applications, it has been disclosed to configure thetracks in order to reduce geometric effects on the rate of corrosion ofthe tracks, it remains necessary to ensure that the thin film materialclosely mimics that of the bulk structure to which the sensor isattached. This is done firstly by using, for the thin film tracks,metallic material having the same composition as the alloy or metal fromwhich the bulk structure is fabricated, and secondly by annealing thethin film in order to ensure that the microstructure of the thin film isat least approximately the same as that of the alloy or metal from whichthe bulk structure is fabricated.

An alternative corrosion sensing strategy is proposed in the paper“Corrosion Sensors in Platform Management” by D. G. Dixon, M. C.Hebbron, S. J. Harris and A. Rezai and presented at the 1^(st) WorldCongress on Corrosion in the Military, 1^(st) June 2005. The authorsrefer to the above-referenced published patent applications, and proposea similar resistive sensor. However, the sensor disclosed by Dixon etal. is covered with a corrosion-inhibiting primer paint that is providedwith an intentional defect in order to mimic, for example, the effect ofa crack. The sensor is located on the structure to be monitored. Atfirst, the defect will be protected from corrosive media by inhibitorspecies leaching from the paint, but, once the reservoir of inhibitor isexhausted, the sensor tracks will corrode, and a corresponding increasein the sensor resistance can be measured. Corrective action can then betaken. Such sensors are also known as “Inhibitor Depletion Sensors”.

In the case of an inhibitor depletion sensor, it is important that theresistive tracks beneath the defects corrode rapidly once theprovisional protective effect of the inhibitor leaching from the painthas been exhausted, so that an operator can be made aware as quickly aspossible that maintenance of the structure may be necessary. Thus thereexists, in both cases, a need to be able to tailor the sensitivity ofany particular type of corrosion sensor. The present invention arose asa result of consideration of the above-identified problems.

SUMMARY OF THE INVENTION

In accordance with a first aspect of the present invention, there isprovided a corrosion sensor comprising: an insulating substrate; a thinfilm of a first metallic material formed on the substrate; and an arrayof areas comprising a second metallic material provided at the surfaceof the thin film. Herein, it should be understood that the term‘metallic’ is used mean both alloys comprising one or more elementarymetal components, and elementary metals. Galvanic corrosion is enhancedat the boundaries between the first and second metallic materials, andthus the sensitivity of the sensor can be tuned by altering the size andspacing of the metallic areas. Preferably, the areas are discrete areas.Routine experimentation can be used to determine the desiredsensitivity, and thus the size and separation of the areas, for anyparticular application of the sensors.

The second metallic material may be more cathodic than the firstmetallic material. In exemplary embodiments described below, the secondmetallic material is selected such that, in use of the sensor, thesecond metallic material is cathodic relative to the first metallicmaterial. Where the second metallic material is more cathodic than thefirst, galvanic corrosion of the first metal occurs. As those skilled inthe art will appreciate, which metallic material of a pair of metallicmaterials is cathodic relative to the other may depend on theenvironment in which the sensor is placed, and thus the first and secondmetallic materials may preferably be selected in dependence on theapplication for which the sensor is to be used.

The areas may be formed on the surface of the thin film. Such anarrangement enables the sensors to be fabricated conveniently bydeposition of a layer of the first metallic material, followed by alayer of areas of the second metallic material.

Preferably, electrical terminals are defined on the thin film such thatan electrical property of the thin film can be monitored. The electricalproperty of the thin film may be the electrical resistance of the thinfilm. The electrical terminals may be protected from the effects ofcorrosion, for example by a layer of paint. The presence of terminalsthus enables the corrosion of the sensor to be monitored remotely bymonitoring of the electrical property. For example, the thin film maydefine a plurality of resistive tracks connecting the electricalterminals, and the areas comprising a second metallic material may beprovided at the surface of the resistive tracks. This allows corrosionto be monitored by monitoring the resistance of the sensor. A layer ofpaint, such as a paint comprising a corrosion inhibitor, may cover thesubstantially the whole of the thin film, and the layer may define aplurality of defects arranged such that at least a part of eachresistive track is exposed by one of the plurality of defects. Such asensor is an improved version of the prior-known inhibitor depletionsensors, that can be used to monitor the protection provided to astructure by a layer of paint.

In one particular embodiment of an inhibitor depletion sensor, the thinfilm defines a plurality of conducting regions separate from theresistive tracks and the electrical terminals, the conducting regionsbeing provided adjacent the resistive tracks. It has been determinedempirically that the separation of the conducting regions from theresistive tracks can also be varied to control the sensitivity of thesensor to corrosive media. Such sensors are therefore more adaptable,since it is possible to vary its sensitivity both by variation of thesize and spacing of the areas, and by variation of the separation of theconducting tracks from the conducting regions.

The areas may have a diameter in the range between 10 μm to 1 mm. Theareas may be spaced apart by a distance no less than their diameter, andin the range between 10 μm to 1 mm.

The first metallic material may be aluminium. Aluminium is commonly usedin aerospace applications. The second metallic material may is copper.Copper is commonly alloyed with aluminium in alloys for use in theaerospace industry. Where the first and second metallic materials arealuminium and copper respectively, the sensor can be used to mimic thebehaviour of an aluminium and copper alloy.

Alternatively, the second metallic material is selected from the groupconsisting of silver and gold. Silver and gold are particularly noblemetals, and thus their use as the second metallic material would resultin particularly rapid corrosion of the first metallic material.

More generally, where the sensor is for use in monitoring the effects ofcorrosion on an alloy, the first metallic material may be the majorconstituent of the alloy, and the second metallic material may be aminor constituent of the alloy. The second metallic material may be aminor constituent that forms a second phase precipitate in the alloy.Galvanic corrosion can be accelerated around particles of a second phaseprecipitate in an alloy, and this effect can be mimicked on the sensorby appropriate selection of the first and second metallic materials.Notably, this obviates the need for deposition of a material mimickingthe alloy. Instead, pure metals can be deposited, with the effect of thesecond phase precipitate mimicked by areas of the second phaseprecipitate metal deposited onto the thin film of the first metallicmaterial.

The invention extends to a vehicle comprising a plurality of sensors asdefined above.

In accordance with a second aspect of the present invention, there isprovided a method of manufacturing a corrosion sensor comprising thesteps of:

(i) providing an insulating substrate;

(ii) depositing a thin film of a first metallic material onto theinsulating substrate; and

(iii) depositing areas of a second metallic material onto the thin film.

The method may further comprise the step of: (iv) removing material fromthe thin film to define resistive tracks. Conveniently, the step ofremoving material comprises removing material using photolithography.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and further features of the invention are set forth withparticularity in the appended claims and will be described hereinafterwith reference to the accompanying drawings in which:

FIG. 1 is a schematic view corrosion sensor in accordance with a firstembodiment of the invention;

FIG. 2 is a schematic view of a corrosion sensor in accordance with asecond embodiment of the invention;

FIG. 3 a is a schematic view of a corrosion sensor in accordance with athird embodiment of the invention;

FIG. 3 b is a detailed view of a part of the corrosion sensorillustrated in FIG. 3 a; and

FIG. 4 is a schematic view of a corrosion sensor in accordance with afourth embodiment of the invention.

DETAILED DESCRIPTION

The embodiments of the present invention described below areparticularly suited to the monitoring of corrosion in alloys. Alloys arematerials comprising one or more metallic components. Typically, analloy will have one major component, and one or more minor componentsthat are present in smaller quantities than the major component. Forexample, the 2000 series of aluminium alloys, widely used in theaerospace field, consist primarily of aluminium and copper, with copperpresent at a concentration of around 5% by weight. Other alloyingconstituents, such as magnesium, manganese, silicon, zinc, iron andchromium, may also be present in concentrations of less than 2% byweight. As another example, the 4000 series of aluminium alloys have asilicon constituent proportion forming in the region of 5%-20%, of theweight of the alloy.

It is possible to obtain a solid solution of copper in aluminium, suchthat a bulk sample of an aluminium and copper alloy can have ahomogenous structure. However, more normally, a part of the copperconstituent will be found as a second phase precipitate in the bulkalloy, forming crystallites of copper in a majority phase ofaluminium/copper solid solution. The 2000 series of aluminium alloysexhibits this type of structure. The presence of the crystallites canimprove the mechanical properties of the bulk alloy. However, the secondphase precipitate can also affect the properties of the resulting alloyin the presence of corrosive media, such as sea water, or a salt spray.Galvanic corrosion occurs wherever two dissimilar metals areelectrically connected in the presence of an electrolyte. One of themetals corrodes, whilst the other metal does not. The metal thatcorrodes is less noble than the metal that does not. Other terms arealso used in the art instead of ‘more noble’ and ‘less noble’. Forexample, metals are also described as ‘less active’ and ‘more active’;or as ‘anodic’ and ‘cathodic’. The more active or cathodic metal in anyone electrically-connected pair of metals is that which corrodes is thepresence of an electrolyte. Notably, in any pair of electricallyconnected metals, which metal is more noble than the other will dependon the electrolyte, or corrosive medium, to which the pair of metals isexposed. Considering specifically the 2000 series of aluminium alloys,copper is a more noble metal than aluminium in the presence of seawater, or spray from sea water. Thus the presence of copper crystallitesin electrical contact with an aluminium/copper solid solution results inlocalised galvanic corrosion of the aluminium alloy, and can lead to thedevelopment of corrosion pits.

Corrosion sensor 100 illustrated in FIG. 1 of the accompanying drawingsis particularly designed for a 2000 series aluminium alloy. Sensor 100illustrated in FIG. 1 of the accompanying drawings is of the inhibitordepletion type described in the above-referenced paper “CorrosionSensors in Platform Management” by D. G. Dixon, M. C. Hebbron, S. J.Harris and A. Rezai, and comprises a substrate 110, a patternedconductive thin film 120, and paint substantially covering the substrate110 and the patterned conductive thin film 120. The area covered by theconductive thin film is indicted by dense shading, whilst that coveredby paint is indicated by more sparse shading. The film 120 is arrangedin a ladder-like configuration such that there are three tracks 122,123, 124 running between common terminals 126 and 127. Three defects132, 133, and 134 are introduced into paint 130, with each defect beingprovided at the location of one of the tracks 122, 123, and 124. Thedefects are areas where the protective layer of paint has been removed.However, in the case of an as-fabricated sensor, these areas remaintemporarily protected from the effects of corrosion because of theeffect of inhibitor ions leaching out of the surrounding paint and ontothe area of the defect. The tracks 122, 123, 124 are arranged to bewider than their respective defects 132, 133, 134 such that the entireexposed area beneath each defect consists of metallic track material.Each defect is of a different size. For example, the defect widths canbe chosen to be 4 mm, 0.5 mm, and 0.05 mm.

The conductive thin film 120 is formed on the substrate 110 bysputtering. In the present embodiment, where the sensor is forapplication to a structure formed of a 2000 series aluminium alloy, thefilm is formed of aluminium. The film is patterned either by maskingduring sputtering, or by photolithography subsequent to the depositionof the thin film. An array of copper dots 150 is then deposited, againby sputtering, on each of the tracks 122, 123, and 124. The copper dotsform an array of areas of a different metallic material to thealuminium. The dots may be circular, as shown in FIG. 1, but could be ofany desired shape—for example, it may be preferred to deposit hexagonaldots. The dots 150 act as centres for galvanic corrosion, mimicking theeffect of copper crystallites in the bulk alloy, and enhancing thesensitivity of the tracks 122, 123, and 124 to corrosion.

In the present embodiment, the sensitivity to corrosion of the aluminiumthin film 120 can be tailored by appropriate choice of the size andspacing of the copper dots 150. The selection of size and spacing of thedots in the array 150 can be readily made by those skilled in the art byexperimental comparison of the rate of corrosion a number of samples ofthin film material 120, each having a different dot size and spacing tothe others, with a sample of the bulk material. The diameter and spacingof the dots may be in the range 10 μm to 1 mm; for example, the diameterof the dots may be 250 μm, and their spacing may be 1 mm.

Paint is applied over the conductive thin film using a mask to definethe defects 132, 133, and 134. Alternatively, chemical and dry etchingtechniques are possible. Suitable chemical etchants and masks can bechosen in dependence on the type of paint used. The paint is of the sametype as that covering the bulk structure which the corrosion sensor 200is to monitor, or may be of a type mimicking that covering the bulkstructure. In the present embodiment, a 25 μm thick coating of PR205, aprimer paint commercially available from PRC DeSoto, is used. PR205 is ahigh-solids, chromate-loaded, epoxy-based primer. A suitable topcoat,such as HP03682, also available from PRC DeSoto, is then applied. Oneetchant that can be used for these paints is ethylene glycol. Usingthese techniques, defects having widths between 50 μm and 8 mm can befabricated.

In use, the resistance of the sensor 100 between points 150 is monitoredover time. The resistance of the sensor remains approximately constantuntil the protective effect of inhibitor leaching from the paint aroundthe largest defect 134 ceases, because of exhaustion of the reservoir ofinhibitor ions in the primer paint. At this point, the resistance willbegin to increase as a result of corrosion depleting the amount ofmaterial in the conductive track 124. This, in turn, indicates whencorrosion will begin to affect metal in other parts of the structurewhere there may be similar defects. By introducing a number ofdifferently-sized defects, each over one rung of the ladder, a measureof the continuing effects of corrosion is obtained: the track beneaththe largest defect 124 will corrode first, followed by that under theintermediate-sized defect 123, followed by that under the smallest-sizeddefect 122.

It will therefore be appreciated that it is convenient for tracks 122,124, and 124 to corrode in a manner similar to, or more rapidly than,that of the bulk alloy the corrosion of which the sensor is intended tomonitor. The application of copper dots 150 enables the rate at whichthe corrosive thin film tracks corrode to be adjusted so as either tomore closely mimic the behaviour of the bulk alloy, or so as to corrodemore rapidly than the bulk alloy.

The application of copper dots to thin film tracks can be used to tunethe sensitivity of a number of different types of corrosion sensor thatrely on measurements of the resistance of a thin film of aluminium oraluminium alloy, in generally the same manner as is described above withreference to the first embodiment 100. A corrosion sensor 200 inaccordance with a second embodiment of the invention is illustrated inFIG. 2. Corrosion sensor 200 functions in broadly the same manner ascorrosion sensor 100 illustrated in FIG. 1, and is fabricated in amanner similar to that described above with regard to corrosion sensor100. Corrosion sensor 200 is also intended for the monitoring of apainted structure formed from an aluminium alloy with a predominantminor component of copper. Sensor 200 comprises a substrate 210 and apatterned conductive thin film 220, 230. Film 220 is arranged in aladder-like configuration such that there are three tracks 222, 223, 224(that form the “rungs” for the ladder) running between common terminals226, 227 (that form the “legs” for the ladder). In use of the sensor,electrical connections are made to the sensor from interrogatinginstrumentation via the common terminals 226, 227. In addition to theladder-like configuration 220, there are also provided separateconducting regions 230. These conducting regions are located betweenconducting tracks 222 and 223, between conducting tracks 223 and 224,above (as shown in FIG. 2) conducting track 222, and below (as shown inFIG. 2) conducting track 224. The separate conducting regions 230 arepositioned closely adjacent the conducting tracks 222, 223, 224, withoutmaking electrical contact with the ladder like configuration 220. Thus,there is a conducting region on either side of each conducting track,with a small gap between the conducting track and each conductingregion. There is a further small gap between the conducting regions 230and the common terminals 226, 227. The gap on either side of theconducting track is of uniform width along the entire length of theconducting track, except for those parts of the conducting tracks closeto the common terminals 226, 227.

As for corrosion sensor 100 illustrated in FIG. 1, the thin filmconducting tracks are made of aluminium, and copper dots 250 are appliedto the thin film conducting tracks 222, 223, 224, and the thin filmconducting regions 230, after their deposition. The copper dots, as inthe case of the first embodiment, serve to mimic the effect of coppercrystallites present in the bulk alloy, and enhance the rate of galvaniccorrosion of the thin film. The dots are of a size in the range 10 μm to1 mm, and are spaced apart by a distance in the range 10 μm to 1 mm. Asdescribed above with regard to the first embodiment, the size andspacing of the dots 250 can be varied in order to tune the sensitivityof the sensor 200 to corrosion, such that it matches the sensitivity ofa bulk sample. The copper dots 250 are provided on both the conductingtracks 222, 223, 224, and the conducting regions 230, so that theproperties of those parts of the surface exposed to the ambientcorrosive environment remain uniform.

Paint is applied to the surface of the sensor once the thin film andcopper dots have been deposited, and covers the surface of the sensor200 except for defects 242, 243, and 244 over each of the conductingtracks 222, 223, and 224, and extending over part of the conductingregions 230. The extent of the paint coverage, as in FIG. 1, isillustrated in FIG. 2 by the sparse shading, whilst the location of themetallic thin film is illustrated by the more densely-shaded regions.The paint used for sensor 200 is the same as that used for sensor 100.

Sensor 200 is used in a manner very similar to sensor 100: theresistance of the sensor is monitored to determine when the protectiveeffect of inhibitor leaching from the paint over the defects 242, 243,and 244 ceases. The main advantages of sensor 200 over sensor 100 areits higher resistance, due to the smaller cross-sectional area of theconducting tracks 222, 223, 224 in comparison to tracks 122, 123, 124 ofsensor 100; and the fact that the tracks 222, 223, 224 will corrodecompletely through—whereas tracks 122, 123, 124 will not, since theiredges remain partly protected by paint—so that a larger change inresistance occurs for sensor 200 than for sensor 300 when provisionalprotection ceases. Moreover, the sensitivity of the sensor 200 tocorrosion can be adjusted both by the variation of the size and spacingof the dots 250, and by the spacing of the gaps between the conductingregions 230 and the conducting tracks 222, 223, and 224: it has beenfound empirically that the time to corrosion of the tracks 222, 223, and224 decreases as the gap width is decreased.

In each of the above described embodiments, the presence of copper dotsformed on the surface of a thin film is used to adjust the sensitivityof a thin film of metallic material to corrosion. As will be appreciatedby those skilled in the art, in the case of inhibitor-depletion sensorssuch as the corrosion sensors 100 and 200 illustrated in FIGS. 1 and 2,it may be desired that the corrosive tracks corrode through rapidly oncethe protective effect of leaching inhibitor has ceased. Therefore, itmay be convenient to form a high density array of copper dots on thethin film corrosive tracks in order to accelerate their corrosion. Analternative manner in which the sensitivity to corrosion of the thinfilm track can be enhanced is by careful selection of the material usedfor the dots: the more noble the metal used to form the dots, thestronger will be the enhancement in the rate of galvanic corrosion ofthe surrounding thin film. Thus, where a high rate of galvanic corrosionof the thin film material is desired, a strongly cathodic metal may beused to form the dots, such as silver or gold.

A corrosion sensor 300 in accordance with a third embodiment of theinvention is illustrated in FIGS. 3 a and 3 b. Sensor 300 is fabricatedfor the monitoring of corrosion to a 2000 series aluminium alloy and isa simple resistance sensor, of the type described in European PatentApplication Publication Number 1554563. As shown in FIG. 3 a, sensor 300comprises a conductive thin film 320 deposited on an insulatingsubstrate 310. Conductive thin film forms common terminals 326, 327, anda plurality of conducting tracks 322 running between the commonterminals. The conductive tracks are exposed to the ambient corrosiveenvironment, and not covered by any protective paint (in contrast tosensors 100 and 200 described above). Corrosion is monitored bymonitoring the electrical resistance between the common terminals 326,327. The resistance of the sensor increases as the tracks corrode in thecorrosive environment. Because a number of tracks 322 are includedbetween the common terminals 326, 327, and the tracks are arranged inparallel, the effect of localised corrosion, such as pitting corrosion,can be monitored as an average across all the tracks 322. Moreover, thegeometry of each of the tracks 322 is carefully controlled in order toensure that geometric effects do not appreciably affect the rate ofcorrosion of the thin film 320, and so that the resistance of the sensor300 can be related to the effect of corrosion on the bulk structure.This carefully controlled geometry is described in detail in EuropeanPatent Application Publication Number 1554563.

A section of one exemplary track 322 is shown in FIG. 3 b. Track 322 isof substantially constant width W, and is formed to meander across alinear corridor 340 defined on the substrate 310. The corridor 340 has awidth D2, within which the meanders of corrosive track 322 arecontained. The track formed of a series of generally U-shaped bendsalternately of opposite curvature, resulting in a repeating serpentineshape within the linear corridor 340. Corrosive track 322 is formed ofaluminium, and copper dots 350 are formed on the surface of thecorrosive tracks 322, as is shown most clearly in FIG. 3 b, in order tomimic the effect of copper crystallites in the bulk alloy that enhancethe rate of galvanic corrosion. As in the first and second embodimentsdescribed above, the size and spacing of the dots can be selected suchthat the response of the corrosive tracks 322 to corrosive media mimicsclosely the response of the bulk aluminium alloy to corrosive media.Those skilled in the art will appreciate that the careful adjustment ofthe sensitivity to corrosion of sensor 300 is more important that thatof sensors 100 and 200 described above. In sensors 100 and 200,deterioration in the condition of the paint is sensed, and it isdesirable for the sensors to produce a measurable response as soon asthe protective effect of the paint ceases: thus, it is of limitedconsequence if the corrosive tracks of sensors 100 and 200 corrode morerapidly than the bulk structure. However, for sensor 300, it is thecorrosion of the tracks 322 themselves that provides the measurableresponse of the sensor to corrosion, and it is therefore more importantthat the sensor accurately reflects the condition of the bulk structure.The provision of dots 350 enables the sensitivity of the tracks 322 tocorrosion to be carefully adjusted, by routine experimentation todetermine the appropriate size and spacing of the dots 350.

Fabrication of sensor 300 is performed as described for the prior knownsensors disclosed in European Patent Application Publication Number1554563. It is to be noted that this method of fabrication includes astep of annealing the as-sputtered conductive thin films. In order toimprove the degree to which the corrosive characteristics of the thinfilm tracks mimic the bulk alloy, the thin film layer is annealedfollowing sputtering to encourage growth of metallic grains within thethin layer to produce a thin film which is essentially a two-dimensionalarray of metallic grains, in order to ensure that the conductive thinfilm on the sensors has a similar microstructure to that of the bulkstructure which the sensor is designed to monitor. In the case of sensor300, this step can be omitted, since the sensitivity to corrosion of thethin film can be altered by alteration of the size and spacing of thecopper dots 350.

It is to be noted that, for each of the corrosion sensors 100, 200, 300described above, electrical terminals (not shown in the figures) areformed on the common terminals (126, 127; 226, 227; 326; 327) such thatthe sensors can be connected to external apparatus in order to measurethe resistance of the sensors. Such terminals are areas where electricalconnections, for example by soldering or otherwise bonding wire to thethin film, can be made to the sensors. It is also to be noted that, ineach of the above-described embodiments, the substrate can be formedfrom any suitable insulating material on which a thin film of the trackmaterial can be deposited. For example, Mylar™, or polyimide can beused. Alternatively, a conducting substrate coated with an insulatinglayer of, for example, polyimide can be used.

The use of dots of a second metallic material formed on the surface of athin film of a first metallic material can also be applied to corrosionsensors other than those that use electrical measurements to determinethe degree of corrosion affecting a structure. A corrosion sensor 400 inaccordance with a fourth embodiment of the invention, also for use inmonitoring the effect of corrosive media on a 2000 series aluminiumalloy, is shown in FIG. 4. Sensor 400 comprises a thin film of aluminiumdeposited on a substrate 410, and an array of copper dots 450 formed onthe surface of the thin film. Dots 450 are formed of a size and at aspacing similar to those described above with reference to the first,second and third embodiments of the invention. However, corrosion sensor400 does not require electrical interrogation in order to determine anelectrical property of the thin film, such as its resistance, butinstead is designed to be visually inspected, either by microscope(where the dots are of a small diameter—for example 10 μm), or by eye(where the dots are of a larger diameter—for example 1 mm). It isenvisaged that such an arrangement may be particularly useful in thecase where it is desired to test a variety of samples of sensors in thelaboratory in order to determine the best possible size and spacing forthe dots on a sensor such as sensors 100, 200, or 300 described above;although those skilled in the art will appreciate that such sensorscould also be of use in other environments.

Sensor 400 is fabricated by deposition of a thin film layer of aluminiumonto a substrate, preferably by sputtering. The substrate can be of anyconvenient material that is suitable for use in a sputtering chamber—itis not necessary for the substrate to be insulating in the case ofsensor 400. The array of dots 450 is then deposited on the surface ofthe thin film layers, again preferably by sputtering, with a mask todefine the size and spacing of the dots.

The thickness of the corrosive tracks is selected in accordance with thematerial from which the tracks are formed and the type of applicationfor the microsensor. For example, for monitoring components in a marineenvironment the rate of corrosion is relatively high, and therefore arelatively thick film is used, for example, in the case of an aluminiumalloy, corrosive tracks in the region of 50 μm to 500 μm in thicknessare used. However, for other applications in which the environment inwhich the microsensor is to be placed is less corrosive, highersensitivity to corrosion is required, and therefore thinner films areused to form the corrosive tracks. In the case of monitoring non-marineaircraft components, the thickness of the corrosive tracks is preferablybetween 0.5 μm and 10 μm, for example approximately 1.5 μm. Thethickness of the dots in each of the above-described embodiments issimilar to the thickness of the as-sputtered thin film.

Having described the invention with reference to various specificembodiments, it is noted that these embodiments are in all resectsexemplary. Variations and modifications are possible without departingfrom the scope of the invention, which is defined in the accompanyingclaims. Such variations and modifications will be immediately obvious tothose skilled in the art. For example, whilst in the above, it has beendescribed to use straight conducting tracks between conducting terminalsof sensors 100 and 200, it will be understood by those skilled in theart that it is possible to use tracks of a different configuration, suchas the serpentine tracks described with regard to corrosion sensor 300in accordance with the third embodiment of the invention. In the case ofsensor 200, optionally, such serpentine tracks could be used inconjunction with complementarily shaped conducting regions. The shape ofthe dots applied to the thin film of aluminium can also be modified. Itmay be preferred to deposit, for example, hexagonal-shaped dots. Theshape of the dots may be chosen for convenience for a given fabricationtechnique. Moreover, it may be desired to deposit stripes of copperacross resistive tracks (particularly in the case of sensors 100 and200) in order to ensure that a break in the resistive track is formedquickly, resulting in a large change in the resistance measured acrossthe electrical terminals. Furthermore, whilst it has been describedabove that the dots are preferably of a uniform size and spacing, it isnoted that both regular and irregular arrays of dots are envisaged,since it is currently thought that the effect of the dots on theproperties of the alloy will be dominated by their average size andspacing. Regular arrays of dots may be formed based on a number ofdifferent patterns, for example to improve the rotational symmetry ofthe sensor.

Furthermore, whilst in the above it has been described that the thinfilms and dots of corrosion sensors 100, 200, 300 and 400 are formed ofaluminium and copper respectively, and that the sensors are for use inmonitoring the effects of corrosion on 2000 series aluminium alloys, itis to be noted that the present invention can readily be applied usingother metallic materials, in dependence on the particular platform onwhich the sensors are to be used, the materials from which that platformis made, and the environment in which it is to be operated. The 2000series aluminium alloys are in widespread use in aircraft, and so thesesensors are expected to find application in health monitoring systemsfor aircraft. However, it is anticipated that, by substitution ofappropriate metals for the aluminium and copper in sensors 100, 200,300, and 400, embodiments of the invention could be made that areapplicable to platforms made from different materials. In most cases, itis considered desirable that the metal from which the dots arefabricated is more noble than the metal from which the film isfabricated. In this way, it is the film that corrodes, and not the dots,resulting, in the case of sensors 100, 200, and 300, in a measurablevariation in the conductivity of the sensor. In such examples, if thedots are less noble than the film, the dots will corrode, and there willbe little measurable effect on the resistance of the sensor. In the caseof sensor 400, such considerations are not relevant, it being necessaryonly that visual inspection of the sensor reveals any effects ofcorrosion.

The determination of which metal of a pair of metals is the more noblein the presence of a particular electrolyte can be made with referenceto a galvanic series. Galvanic series list metals in order of thenobility as measured in the presence of a particular electrolyte. Oneexemplary galvanic series is set out in MIL-STD-889, a military standardpublished by the US Department of Defence entitled “Dissimilar Metals”,and lists a number of metals (including alloys) in order of theirnobility in the presence of sea water, from the most noble to the leastnoble. Where pairs of metals selected from those listed are inelectrical contact in the presence of sea water, the metal closest tothe ‘active’ or ‘less noble’ end of the list will galvanically corrode.

Of course, it may be desirable to use a second metallic material for thedots that is different to any alloying constituent, in order to enhancethe corrosion rate of the thin film—for example, a noble metal such asgold could be used for the dots.

It is also possible to modify the sensors 100, 200, 300 as describedabove such that the substrate is almost entirely covered with theconductive thin film. Such a modification ensures that the paint willstick to the sensor more uniformly. As those skilled in the art willappreciate, paint will not stick to the material of the substrate to thesame degree that it will adhere to the conductive thin film, andtherefore, by ensuring that the substrate is substantially entirelycovered with the conductive film (leaving only gaps to define theconducting tracks 222, 223, 224 and terminals 226, 227, in the exampleof sensor 200), it can be ensured that the sensor mimics more closelythe behaviour of the bulk structure.

Those skilled in the art will also appreciate that, whilst it has beendescribed in the above to use commercially available paints for sensors100 and 200, it may be necessary, where such sensors are to beretro-fitted to existing vehicles, to mix paint with an appropriateinhibitor ion concentration, in order to mimic the effect of the age ofthe paint on the structure to be monitored. It is also noted that paintscomprising corrosion inhibitors are widely available from a number ofmanufacturers, including Akzo Nobel, Anac, and Indestructible Paints,which manufacturers are able to supply equivalents to the PR205 primerpaint used in the above-described embodiments of the invention.Moreover, it is also envisaged to use paints comprising corrosioninhibitors other than chromate ions. Such paints are expected to becomemore widely used in the future because of the potential hazards ofchromate-containing paints.

Those skilled in the art will also appreciate that any discrete areas ofa second metallic material may be used instead of dot-like areas. It maybeHowever, sensors in which areas other than dots of the second metallicmaterial are used are envisaged, and in such cases it may be desirablefor the second material to be less noble than the first.

Finally, it is noted that it is to be clearly understood that anyfeature described above in relation to any one embodiment may be usedalone, or in combination with other features described, and may also beused in combination with one or more features of any other of theembodiments, or any combination of any other of the embodiments.

The invention claimed is:
 1. A corrosion sensor comprising: aninsulating substrate; a thin film of a first metallic material formed onthe substrate, the thin film constructed and arranged in a ladder-likeconfiguration including a plurality rung-like features; an array ofseparate areas comprising a second metallic material, positioned on theplurality of rung-like features; a plurality of electrical terminalsdefined by the thin film for monitoring electrical resistance of thethin film; and a plurality of resistive tracks defined by the rung-likefeatures of the thin film, at least portions of the plurality ofresistive tracks providing a plurality of separate electrical pathwaysbetween the electrical terminals, wherein the separate areas comprisingthe second metallic material are provided at surfaces of the portions ofthe plurality of resistive tracks that provide the plurality of separateelectrical pathways for enhancing a rate of galvanic corrosion of thethin film.
 2. A corrosion sensor as claimed in claim 1, wherein thesecond metallic material is selected such that the second metallicmaterial will be cathodic relative to the first metallic material duringsensor operation.
 3. A corrosion sensor as claimed in claim 1, whereinthe electrical terminals are protected from effects of corrosion.
 4. Acorrosion sensor as claimed in claim 3, wherein the electrical terminalsare protected from the effects of corrosion by a layer of paint.
 5. Acorrosion sensor as claimed in claim 1, wherein a layer of paint coversthe thin film, the layer defining a plurality of defects arranged forexposing at least a part of each resistive track by one of the pluralityof defects.
 6. A corrosion sensor as claimed in claim 5, wherein thepaint comprises a corrosion inhibitor.
 7. A corrosion sensor as claimedin claim 6, wherein the thin film defines a plurality of conductingregions separate from the resistive tracks and the electrical terminals,the conducting regions being provided adjacent the resistive tracks. 8.A corrosion sensor as claimed in claim 1, wherein the areas have adiameter in the range between 10 μm to 1mm.
 9. A corrosion sensor asclaimed in claim 1, wherein the areas are spaced apart by a distance noless than their diameter, and in a range between 10 μm to 1mm.
 10. Acorrosion sensor as claimed in claim 1, wherein the first metallicmaterial is aluminum.
 11. A corrosion sensor as claimed in claim 1,wherein the second metallic material is copper.
 12. A corrosion sensoras claimed in claim 1, wherein the second metallic material is selectedfrom the group consisting of silver and gold.
 13. A corrosion sensor asclaimed in claim 1 for monitoring. effects of corrosion on an alloy,wherein the first metallic material is a major constituent of the alloyto be monitored, and wherein the second metallic material is a minorconstituent of the alloy.
 14. A corrosion sensor as claimed in claim 13,wherein the second metallic material forms a second phase precipitate inthe alloy.
 15. A corrosion sensor as claimed in claim 1, wherein eacharea in the array of separate areas is a dot.
 16. A corrosion sensor asclaimed in claim 1, wherein at least some of the resistive tracks of theplurality of physically separate electrical pathways have differentsensitivities to corrosion.
 17. A corrosion sensor as claimed in claim1, wherein the array of separate areas comprising the second metallicmaterial includes areas of different sizes.
 18. A vehicle comprising aplurality of corrosion sensors, each corrosion sensor comprising: aninsulating substrate; a thin film of a first metallic material formed onthe substrate the thin film constructed an arranged in a ladder-likeconfiguration including a plurality of rung-like features; an array ofseparate areas comprising a second metallic material positioned on theplurality of rung-like features; a plurality of electrical terminalsdefined by the thin film for monitoring electrical resistance of thethin film; and a plurality of resistive tracks defined by the rung-likefeatures of the thin film, at least portions of the plurality ofresistive tracks providing a plurality of separate electrical pathwaysbetween the electrical terminals, wherein the separate areas comprisingthe second metallic material are provided at surfaces of the portions ofthe plurality of resistive tracks that provide the plurality of separateelectrical pathways for enhancing a rate of galvanic corrosion of thethin film.
 19. A corrosion sensor as claimed in claim 18, wherein thearray of separate areas comprising the second metallic material includesareas of different sizes.
 20. A method of manufacturing a corrosionsensor, comprising: providing an insulating substrate; depositing a thinfilm of a first metallic material onto the insulating substrate, thethin film constructed and arranged in a ladder-like configurationincluding a plurality of runt like features; defining a plurality ofelectrical terminals on the thin film for monitoring electricalresistance of the thin film; defining a plurality of resistive tracks onthe rung-like features of the thin film at least portions of theplurality of resistive tracks providing a plurality of separateelectrical pathways connecting the electrical terminals; and depositingareas of a second metallic material onto surfaces of the portions of theplurality of resistive tracks on the rung-like features that provide theplurality of separate electrical pathways for enhancing a rate ofgalvanic corrosion of the thin film.
 21. A method as claimed in claim20, comprising: removing material from the thin film to define theresistive tracks.
 22. A method as claimed in claim 21, wherein theremoving of material comprises: removing material usingphotolithography.
 23. A method as claimed in claim 20, wherein the areasof the second metallic include areas of different sizes.